Thermionic gaseous discharge rectifier



ZJQSEEM Sept S, WEQQ P. L. SPENCER THERMIONIC GASEOUS DISCHARGERECTIFIER Filed Feb. 27, 1955 INVENTOR PERCY LSPENCER BY 82 ATTORNEYPatented Sept. 8, 1936 UNITED STATES 2,053,501 THERMIONIG GASEOUSDISCHARGE RECTIFIER Percy L. Spencer, West Newton, Masa, assignor toRaytheon Production Corporation, Newton, Mass., a corporation ofDelaware Application February 27, 1935, Serial No. 8,435 6 Claims. (01.250-2'7.5)

This invention relates to thermionic rectifiers, and more particularlyto such rectifiers in which the cathode is heated to temperature ofthermionic emission solely by means of the discharge 55 current.

An object of this invention is to devise such a rectifier which may besubjected to a large number of starting operations and still possess alife sufiiciently long for commercial purposes.

Another object of my invention is to produce such a device in which thevoltage drop is low.

A further object of my invention is to devise a simple, effective anddurable structure which will accomplish each of the above purposes.

The foregoing and other objects of my invention will be best understoodfrom the following description of an exemplification thereof, referencebeing had to the accompanying drawing wherein the figure is a side viewof an embodiment of my invention, with certain parts thereof incross-section.

In gaseous space discharge devices using thenmionic cathodes, suchcathodes are usually heated to temperature of thermionic emission byheating 1% current passed through the cathode by an independent sourceof current or by a separate heating element. In order to simplify thistype of device, it is desirable to eliminate the independent heatingarrangement and cause the discharge current which passes through thedevice to raise the cathode to temperature of thermionic emission aswell as maintain it at said temperature during operation. It is furtherdesirable that the above simplification be accomplished without thesacrifice of low voltage drop and consequently relatively highefficiency which is obtained with the independently heated type ofcathode. In order to obtain such low voltage drop, it is desirable tohave the cathode coated with materials whichemit electrons at relativelylow temperature, such as, for example, the oxides of the alkaline earthmetals. If it is attempted to heat a coated cathode to temperature ofthermionic emission by the discharge in which the starting 5 current isallowed to impinge upon substantially the entire coated cathode, thecoating is subjected to severe bombardment by relatively fastmovingpositive ions, which in a relatively short time destroys the coating andends the useful life of the tube. In accordance with my invention I havediscovered that the desired results can be secured if a small coatedcathode is used, of which the major portion is protected during startingfrom bombardment by positive ions and 55 the starting current is allowedto impinge only upon a projection on said cathode arranged so that thisstarting current can heat said cathode to temperature of thermionicemission.

In the drawing I have shown a rectifier incorporating the aboveprinciples and consisting of a 5 glass envelope I having a reentrantstem 2 with a press 3 at the upper end thereof, said press carrying acathode 4 and a plurality of anodes 5.

The cathode 4 consists of a few turns of small refractory metal wire,such as, for example, tan- 1o talum wire. In a practical embodiment ofmy invention I have used tantalum wire of .0"3 inch diameter. Thecathode 4 carries at its upper end a projection 6 which is acontinuation of the wire forming the turns of said cathode. The 15cathode is coated with an electron emissive coating, preferably in themanner as will be described below. Closely surrounding the turns of thecathode is a shielding sleeve 1 made of any suitable metal, such as, forexample, nickel. This 6 sleeve I is supported on the press 3 by means ofa tubular projection 8 formed on said press. The projection 5 extendsfrom the cathode beyond the upper end of the shield I.

Each anode 5 is made of a short rod of a re- 5 fractory conductingmaterial, such as, for example, graphite or tantalum. Each anode 5 issupported by an insulating sleeve 9, preferably of lava, which in turnis supported by a tubular glass projection l0 formed on the press 3. The30 cathode 4 isprovided with a lead-in conductor l I which is sealed inthe press 3 and extends through said press and reentrant stem 2 to theexterior of the tube. Each anode 5 is likewise provided with a lead-inconductor l2 sealed in the press 3 and 35 extending therethrough andthrough the reentrant stem 2 to the entrance of the tube. The tube I maybe provided with a conventional base l3 having a plurality of externalcontact prongs. The anode conductors l2 are connected to corre- 4osponding anode contact prongs l4, and the cathode conductor II isconnected to the cathode contact prong l5.

Although my device may operate satisfactorily with various types ofelectron-emissive cathode coating, I prefer to use a special coatingprocess which produces a cathode surface which is very durable and ableto withstand discharges of arc intensity for relatively long periods. Ipreferably first oxidize the surface of the cathode in 5 any suitablemanner as by heating in a gas flame. Thereafter I coat the cathode witha mixture of solutions of alkali metal compounds, preferably thenitrates of barium, strontium, potassium, and if desired other alkalineearth or alkali metals. These compounds are bln-ned into the surface ofthe metal by an intense flame which is preferably a reducing flame. Ihave found that a flame produced by illuminating gas and air'issatisfactory for this purpose. The burning process is carried out untilthe surface of the cathode appears to become white, encrusted and roughwhile the metal itself becomes brittle. This coating and burning processmay be repeated several times.

The cathode coated in the above manner is mounted in the tube asdescribed above. The tube is then evacuated in accordance with-the usualpractice. During the evacuation the oath-- ode is subjected to furthertreatment to convert the coating thereof to its final form. In order todo this, a discharge is passed through the device sufficient to heat thecathode to a relatively high temperature at which the coating on thecathode is further broken down. Under these conditions the cathode is nolonger brittle, and the coating is presumably reduced to the oxides ofthe alkali metal together with possibly some pure alkali metalimpregnated into the surface of the cathode.

The tube as constructed in the above manner is freed of occluded gasesin accordance with the usual practice, and is then filled with a gas atsufficiently high pressure to produce copious ionization therethroughupon the passage of a discharge through the device. I preferably use arare gas, such as, for example, neon, helium, or argon, or a mixture ofsuch gases at a pressure of about five millimeters of mercury. Due tothe comparatively low ionizing voltage of argon, I prefer to use argonas the gaseous filling.

The tube which I have illustrated may be connected in any suitablecircuit, such as that illustrated diagrammatically in the drawing. Atransformer l6 may have its primary II connected to a source ofalternating current, and its secondary l8 may be connected at itsopposite ends to the two anode prongs M. The center of the secondarywinding may be connected through a suitable load I! to the cathode prongI 5.

In accordance with my present understanding of the theory of operationof the device, the tube operates substantially as follows. When thetransformer I6 is energized, a voltage is impressed upon the device,between each of the anodes 5 and cathode 4. Under these conditions, aglow discharge will start through the device, and the glow current willconcentrate upon the exposed areas of the anode 5 and upon theprojection 6 of the cathode l. The spacing between the shield I and theturns of the cathode is sufliciently close so that this glow dischargecurrent does not impinge upon any part of the cathode except saidprojection 6 The voltage drop which occurs through the tube during thestarting op- ,eration is relatively high, and a considerable amount ofheat is*liberated at the cathode in the vicinity of the projection 6.This heat travels down to the relatively small cathode l, and quicklyheats it to a temperature at which said cathode emits thermionically.Since the turns of the cathode are connected in series with theprojection 6,- the current which flows to said projection 6 likewiseflows through the turns of the cathode l, and this current likewisetends to heat the cathode to its operating temperature. As soon as thecathode is heated. to temperature of thermionic emission, the coatedturns start to emit electrons and the current then no longerconcentrates solely upon the projection 8 but is drawn from the coatedturns of the cathode. Under these conditions the drop through the tubefalls to a low value. In practical embodiments of my invention Ihavesecured voltage drops of about ten volts at a load of about fortymillianaperes. It will be seen that, due to the presence of the shieldI, the main coated portion of the cathode is not sub- .jected topositive ion bombardment during starting, this bombardment beingconfined substantially solely to the projection 6. The fact that anycoating which exists on the projection 8 may be eventually dislodgedtherefrom, due to such bombardment, has no detrimental effect upon theoperation of my device inasmuch as it operates satisfactorily withoutany coating on said pro ection 6. Furthermore, the shield 1 preventsradiation of heat from the coated portion of the cathode, and thereforebut a comparatively small amount of energy is necessary to maintain thecathode at its operating temperature. Thus during operation a negligibleamount of energy is utilized in keeping the cathode hot, and thus thevoltage drop through the tube can be very low and still providesufficient energy to keep the cathode at its proper temperature. It isprobably due to this arrangement that the voltage drop through the tubecan be reduced to the low values which I have been able to obtain withmy device.

This invention is not limited to the particular details of construction,materials or processes as described above, as many equivalents willsuggest themselves to those skilled in the art. It is accordinglydesired that the appended claims be given a broad interpretationcommensurate with the scope of the invention within the art.

What is claimed is:

' 1. An electrical space discharge device comprising an envelopecontaining an ionizable gas, electrodes adapted to support an ionizingdischarge through said gas, one of said electrodes being a thermioniccathode, said cathode having thereon a coating of electron-emissivematerial and adapted to be heated to temperature of thermionic emissionsolely by the discharge, means electrically insulated from said coatedcathode to prevent the glow discharge which occurs during starting fromimpinging on said coated cathode, and a projection electricallyconnected to said cathode and projecting beyond said means on whichprojection the starting glow discharge current impinges.

2. An electrical space discharge device comprising an envelopecontaining an ionizable gas, electrodes adapted to support an ionizingdis charge through said gas, one of said electrodes being a thermioniccathode, said cathode having thereon a coating of electron-emissivematerial and adapted to be heated to temperature of thermionic emissionsolely by the discharge, a shield surrounding said coated cathode andelectrically insulated therefrom to prevent the glow discharge whichoccurs during starting from impinging thereon, and a projectionelectrically connected to said cathode and projecting beyond said shieldon which projection the starting glow discharge current impinges, saidshield having an opening through which the discharge can pass when saidcathode has been heated to temperature of thermionic emission.

3. An electrical space discharge device comprising an envelopecontaining an ionizable gas, electrodes adapted to support an ionizingdischarge through said gas, one of said electrodes being a thermioniccathode, said cathode having thereon a coating of electron-emissivematerial and adapted to be heated to temperature of thermionic emissionsolely by the discharge, a metal tubular shield surrounding said coatedcathode and electrically insulated therefrom to prevent the glowdischarge which occurs during starting from impinging thereon, and aprojection electrically connected to said cathode and projecting beyondsaid shield on which projection the starting glow discharge currentimpinges, said shield having an opening through which the discharge canpass when said cathode has been heated to temperature of thermionicemission.

4. An electrical space discharge device comprising an envelopecontaining an ionizable gas, electrodes adapted to support an ionizingdischarge through said gas, one of said electrodes being a thermioniccathode, said cathode having alkaline earth metal compounds burned intoits surface and adapted to be heated to temperature of thermionicemission solely by the dis-- charge, a shield surrounding said coatedcathode and electrically insulated therefrom to prevent the glowdischarge which occurs during starting from impinging thereon, and aprojection electrically connected to said cathode and projecting beyondsaid shield on which projection the starting glow discharge currentimpinges, said shield having an opening through which the discharge canpass when said cathode has been heated to temperature of thermionicemission.

5. An electrical space discharge device comprising an envelopecontaining an ionizable gas, electrodes adapted to support an ionizingdischarge through said gas, one of said electrodes being a thermioniccathode, said cathe having alkaline earth metal compounds burned intoits surface and adapted to be heated to temperature of thermionicemission solely by the discharge, a shield surrounding said coatedcathode and electrically insulated therefrom to prevent the glowdischarge which occurs during starting from impinging thereon, and aprojection electrically connected to said cathode and projecting beyondsaid shield on which projection the starting glow discharge currentimpinges, said shield having an opening through which the discharge canpass when said cathode has been heated to temperature of thermionicemission, said cathode being sufliciently small and said shieldpreventing loss of radiant energy from said cathode sufliciently toproduce a discharge voltage drop of the order of the ionization voltageof the gas or less.

6. An electrical space discharge device comprising an envelopecontaining argon at a pressure of about five millimeters of mercury,electrodes adapted to support an' ionizing discharge through said argon,one of said electrodes being a thermionic cathode, said cathode havingthereon a coating of electron-emissive material and adapted to be heatedto temperature of thermionic emission solely by the discharge, a shieldsurrounding said coated cathode and electrically insulated therefrom toprevent the glow discharge which occurs tiuring starting from impingingthereon, and a projection electrically connected to said cathode andprojecting beyond said shield on which projection the starting glowdischarge current impinges, said shield having an opening through whichthe discharge can pass when said cathode has been heated to temperatureof thermionic emission.

PERCY L. SPENCER.

